Are they sure Cletus Kasady is back as Carnage? Or is Tanis Nieves still this She-Carnage? Will they keep them both as Carnage? What do you think?   

Yes. Carnage is Cletus. My PROOF? Very little but strong proof. Look at the legs of Carnage in issue 5... what do you see? Metal parts... in the 3rd issue Cletus is revealed to have robotic legs now. As for the big ol' symbiote in the background.... THAT might be Tanis. They wouldn't just introduce her and then kill her off (Though I would find that wicked funny) this could be all the samples from each of the symbiotes taken from the Carnage symbiote to make Royal Blue and ect. (Super Symbiote idea from InnerVenom123)

I was very suprised to see Cletus alive, I remember reading an interview by the writer or something where we were assured he was dead.  I thought that was awsome, I did not expect it at all, and until I saw the picture above it had me wondering, maybe the symbiote is going to kill him.  So obviously now Cletus and symbiote reunite, what who is that in the background?  It could be several things, several of which scare me that it could lead  into more symbiotes, which we don't need, it could lead into bringing Toxin into the picture, which could also be disappointing. What ever they do, I have faith it will be good,  I've been enjoying the read so far and I think it will be good.  I like stories that are a little out of continuity where the writer isn't restricted by a basically unrelated bigger story.

I think he's back as Carnage and that Tanis was merely transporting the symbiote to him

Yes he is back:

oh my god THANK YOU!!!!!!!!!!!!!   AM I RIGHT????